To reduce the cost per watt of power supplies in high power systems, and provide highly reliable, redundant power systems, the industry has been attempting to parallel low cost supplies. Although a "ground-up" design may make it more feasible to develop a totally integrated power system, this is typically the most expensive approach. This is due to the fact that such a power supply will have only limited use. By comparison, lower wattage PC power supplies are made in large quantities, and therefore, the dollars/watt for these power supplies is typically the lowest. In order to reduce cost, the industry has been trying to parallel these low cost power supplies. However, this tends to be difficult in that any modification of the power supplies to provide internal control thereto defeats the cost advantages provided by the power supply itself, and without some type of control, the performance of the combined power supply is less than satisfactory.
One technique that the industry has attempted to utilize is a master/slave combination. This defines one of the power supplies as the master with the remaining power supplies designated as slaves. This provides some redundancies but, if the master fails, the whole system will go down. Another technique is to utilize fixed resistors to balance the loading between paralleled supplies. This has a drawback of poor regulation when one supply fails, as the fixed resistor associated with the failed supply remains in the system, resulting in more current through this resistor, wherein the voltage "droop" could become excessive. This can present a problem in modem day computers in that the central processing chips will not tolerate much voltage droop. This would therefore require additional parallel resistors to be added to reduce this voltage droop, which is difficult to accomplish.
Another technique for paralleling power supplies is to provide a feedback loop that is common to the power supplies However, this method requires customizing the supply and, therefore, raising costs. Of course, there are some supplies that allow for direct parallel operation. These are referred to as "voltage droop" power supplies whereto the output voltage is designed to droop depending upon the output current. However, this type of parallel configuration has problems with tight regulation requirements. Another system, an active voltage sense control system, provides an external paralleling board for sensing the current from each supply and subsequent control of the output by feeding back an external signal on its external sense line. These systems have some disadvantages in that they require remote sensing to be on the supply, thus increasing cost. Further, the control board now constitutes an additional reliability consideration.
Another technique, a passive voltage sense control system, is similar to the active voltage sense control, but it utilizes fixed passive elements in series with the voltage sense lines to force current sharing. This system does not have as tight a regulation as the active voltage sense control method, but it does have some destructive failure sense modes.
An additional cost factor for a power supply is the series sense element, typically a sense resistor, that is required to determine the current through the power supply. Use of such a resistor, in addition to increasing the parts count, also results in the need for higher voltages and unwanted power losses.